I took an online quiz and according to it I am way lacking in calcium, magnesium and potassium. Should I take supplements or a multi vitamin?

You could also get this information by running FitDay's Reports.

Supplements for Calcium and Magnesium are the way to go. You'll never get the RDA's of either without eating a LOT of calories to supply you with these nutrients. But if you want to get them naturally, by all means eat a ton of food.

Potassium should be achieved naturally. I eat a sweet potato every day which supplies not only a huge amount of potassium, but it's got a robust amount of magnesium and a decent amount of calcium as well.

Multivitamins are deficient of Magnesium because Mg inhibits the absorption of calcium, so a separate supplement is needed for sure. It also needs to be timed carefully.

See this for additional suggestions for getting your potassium naturally: